Liquid propane gas is a desirable fuel for engines due to its availability and fewer pollutants upon burning. One difficulty in using liquid propane gas as fuel is that it must be vaporized prior to introduction to the carburetor of an engine. Another difficulty is vaporization of liquid propane gas for low temperature starts.
It has been known that hydrocarbon fuels (for example propane or butane fuel) with boiling points below operating ambient temperature used for spark ignition internal combustion engines must be carburated in a gaseous state. These fuels are most economically stored in liquid state and require vaporization by addition of heat. For reliable and efficient engine operation, the fuel/air mixture must be maintained at prescribed ratios throughout the load, speed and ambient temperature ranges of the engine.
The design of ordinary carburetors, for fuels in gaseous state, assumes that the fuel is maintained at a constant density (i.e., temperature and pressure) to maintain the proper fuel/air ratio. If fuel density increases, the fuel/air ratio increases resulting in a richer fuel mixture. Conversely, if density decreases, the fuel/air ratio decreases resulting in a lean fuel mixture. To help maintain constant fuel density, a pressure regulator is often used. Constant fuel temperature is also required to maintain constant fuel density. Some liquid fuel vaporizer systems suffer performance losses because they either do not control fuel temperature (such as those systems disclosed in U.S. Pat. Nos. 3,184,295 and 2,752,758) Others use a temperature control system such as a thermostat to control fuel temperatures (such as those illustrated in U.S. Pat. Nos. 2,752,758; 4,341,194; 2,073,276 and 4,099,499).
Fuel consumption and the heat flux required for fuel vaporization vary greatly with engine load and speed. To maintain constant fuel temperature, the heat flux supplied by the vaporizer must vary to match the engine's specific requirements. During low ambient temperature starts, a spark ignition engine requires a higher fuel/air ratio (i.e. richer fuel mixture) than when the engine is running at operating temperature. This is typically accomplished with an electric solenoid overriding the demand stage of the pressure regulator.
In systems for water cooled engines only, initial vaporization is supplied by thermoenergy stored in the regulator/vaporizer and in the engine coolant circulated through the regulator/vaporizer once the engine has started. Since liquid propane vaporizes at minus 44.degree. F. and the regulator/vaporizer and engine coolant are at ambient temperatures, less fuel is vaporized during start up. Consequently, in this system, only fuel sufficient to idle the engine is available after start up. After approximately fifteen minutes, the engine and coolant are sufficiently hot such that a full load may be imposed on the engine. With such systems starting the engine is difficult and the engine runs poorly for several minutes after starting. Examples of systems using engine coolant to vaporize fuel are disclosed in U.S. Pat. Nos. 4,341,194; 2,752,758; 2,896,658; 3,184,295; 2,788,779; 2,821,843; 3,114,357 and 3,565,201.
One liquid fuel system passes exhaust gas through passages provided in a vaporizer designed for use with water coolant as a heat source. This raises the temperature of the vaporizer more rapidly and avoids long periods of engine idling before applying a full load to the engine. This system has not been approved by the necessary certifying organizations since the materials used in the vaporizer can fail allowing exhaust and fuel systems to be connected resulting in a dangerous situation.
Other liquid fuel systems are specifically designed to use engine exhaust as a vaporizing heat source. Those systems invariably incorporate a temperature control system to prevent under-vaporization or overheating of the fuel. A temperature control system is required because these systems place the vaporizer significantly ahead of the pressure regulator resulting in a large heat flux time constant during load and speed transients. As a result, these systems produce wide swings in heat flux to compensate for lack of responsiveness to heat flux changes. This results in oversizing of the heat exchanger for transient conditions. Such systems are disclosed in U.S. Pat. No. 4,099,499.
Other systems employ air heated vaporizers for air cooled engines. The cooling air, however, does not warm above ambient temperature for approximately five minutes after start up. Since the amount of heat stored in the regulator drops rapidly as low temperature liquid propane gas flows through the regulator, cold starting capability of this system is severely limited.
To avoid the problems of the above mentioned systems there are systems that employ a heat exchanger mounted on the exhaust gas manifold. All initial heat for vaporization during engine start up is supplied by thermal energy of the regulator. Additional heat is supplied once the temperature of the heat exchanger is elevated by exhaust gas in the manifold. However, these systems require warm up periods before full load can be applied to the engine. Systems employing a heat exchanger heated by exhaust gas are illustrated in U.S. Pat. Nos. 2,409,611; 3,524,734; 3,978,823 and 2,386,594.
U.S. Pat. No. 2,386,594 discloses a fuel vaporizer that is mounted directly on the exhaust manifold of the engine. A system of this type suffers from the disadvantage that there is no control of the temperature of the vaporizer and under heavy loads the exhaust manifold temperature will be extremely high. The vaporizer will be substantially the same temperature as the manifold and this high temperature can result in over-vaporization of the liquid propane gas causing the engine to shutdown.
A variation of this type of device is illustrated in U.S. Pat. No. 2,711,718 in which a vaporizer is disclosed as being mounted on and within the exhaust manifold. This system not only may result in over-vaporization but also is subject to the dangerous condition of failure causing fuel to pass into the exhaust system creating the possibility of an explosion.
A further example of a vaporizer system is one that coils a fuel line around an exhaust pipe. The fuel line is wound so as to touch the exhaust pipe at one or two points. After operation, the fuel line no longer completely touches the exhaust pipe resulting in less vaporization of the liquid propane gas. An example of a device of this type is illustrated in U.S. Pat. No. 1,384,512.
In U.S. Pat. No. 2,073,276, placing a heating coil within a branch of the exhaust pipe or manifold is disclosed. Fuel passing through this coil is vaporized. If a failure occurs in this system, fuel may pass into the exhaust resulting in an explosion. A similar procedure is disclosed in U.S. Pat. No. 2,244,623 in which the fuel line is positioned within the water jacket of the engine.
Another approach to vaporizing liquid fuel is to heat the pressure regulators electrically such as disclosed in U.S. Pat. Nos. 4,092,963 and 2,030,745. The use of an electric heater to vaporize fuel in a gas generator is illustrated in U.S. Pat. No. 1,950,860. An electric vaporizer in proximity to an exhaust manifold is disclosed in U.S. Pat. No. 2,342,132. This system uses both a heated filament and heat from an exhaust manifold to gasify raw liquid fuel. U.S. Pat. Nos. 4,047,512 and 4,092,275 disclose metering systems in which fuel may be electrically heated for vaporization.
Another difficulty in prior art systems is matching the rate of vaporization of fuel to the engine load. As more load is applied to the engine, more fuel is required and a greater rate of vaporization is necessary. One way to increase the rate of vaporization is to increase the temperature of the vaporizer. Raising the temperature has been difficult to accomplish since temperature must rise and fall in proportion to the increase and decrease of the load on the engine. Complicated and expensive systems have been used with less than complete success.